Use launchpad to improve and extend datepicker localizations

We've got a good bunch of localizations already, though there's still a lot of languages missing, and we probably will need some additional terms for the ARIA support (e.g. "selected, "datepicker" labels). Launchpad Translations could be a good way to get there: ​https://translations.launchpad.net/
Alternative is ​https://www.transifex.net/